1. Field of the Invention
The present invention relates to a cutting tool and, more particularly, to a dental rotary instrument which is adapted to be used for cutting teeth or dental restorations, and is also designed to be used for cutting other materials.
2. Statement of the Prior Art
In general, dental rotary instruments are made by attaching hard abrasive grains such as those based on natural or artificial diamond, aluminum oxide or carborundum on a shank molded or otherwise formed into various shapes such as spherical, cylindrical, conical, wheel shapes and others by means of electroplating or brazing. For use, the dental rotary instruments are mounted on a dental turbine or a dental engine, and they are rotated at a high speed while cooled with poured water or sprayed water at the same time.
For dentists, it is preferred to carry out efficient preparation with such dental rotary instruments, and this is also desirable for patients. Required for that purpose are that:
(1) the cutting tools used excel in cutting efficiency, PA1 (2) the removal of debris produced by cutting teeth or dental restorations is satisfactorily effected, and PA1 (3) the removal of heat generated at teeth or dental restorations and the head of the cutting tool in association with preparation is satisfactorily effected.
Now, various dental rotary instruments to meet such requirements have been proposed.
According to one typical example, three spiral grooves are formed on the outer face portion of the head of a shank, which are formed slightly dextrally with respect to the axis of the head. According to another example disclosed in Japanese patent laid-open publication No. 56-31744, a stack of symmetrical beads of Japanese soroban, each of a hexagonal shape in cross-section, is grooved. According to still another example disclosed in Japanese patent laid-open publication No. 58-500280, a groove having a lefthand thread is provided.
Each of the cutting tools as mentioned above poses the following problems.
The first-mentioned example of cutting tools shows excellent cutting efficiency, when the head of its shank has a large outer diameter. If the head has a small outer diameter, however, the area ratio of the hard abrasive grains-attached section in the head of the shank of the cutting tool is so reduced due to the presence of three grooves that the area of contact of that section with teeth or dental restorations is reduced, thus leading to a lowering of cutting efficiency. This tendency become more marked when a low load is applied.
Referring to the second-mentioned example of cutting tools, the area of the hard abrasive grains-attached section to contact flat teeth or dental restorations during preparation is greatly reduced, leading to a lowering of cutting efficiency. In addition, since the portion of the head is machined in the form of a groove, it is required to readjust the thus grooved portion with the use of an ordinary cutting tool which is not grooved.
The third-mentioned example of cutting tools has the disadvantage that, due to the fact that the width of the groove is virtually equal to the width of the hard abrasive grains-attached section, the area of the hard abrasive grains-attached section to contact teeth or dental restorations is decreased to about 1/2 of the area of the head, with the result that the cutting efficiency drops at the same revolutions per minute.